The invention concerns a process for purifying a sulfur dioxide containing gas by washing with an ammonia aqueous solution.
A known process for purifying a sulfur dioxide containing gas comprises (a) contacting the gas with ammonia and/or ammonium sulfite, in the presence of water, to form an ammonium sulfite and/or bisulfite solution, (b) reacting the solution obtained from step (a) with ammonium bisulfate so as to liberate gaseous sulfur dioxide and to form ammonium sulfate in aqueous solution, (c) recovering sulfur dioxide (d) thermally decomposing ammonium sulfate to separately obtain ammonia and ammonium bisulfate, (c) feeding the ammonia to step (a) in the form of ammonia or ammonium sulfite and (f) feeding the ammonium bisulfate to step (b). See for example U.S. Pat. No. 2,405,747.
However, a problem is posed by the accumulation of ammonium sulfate. As a matter of fact, as stated in the U.S. Pat. No. 3,321,275, ammonium sulfate is formed by absorption of SO.sub.3, always present in admixture with SO.sub.2, during step (a) or by oxidation of the ammonium sulfite solution. This excess sulfate (after conversion to bisulfate) may be decomposed by reduction as proposed in the above mentioned U.S. patent.
This has the disadvantage of consuming a reducing agent and of making it necessary to carry out the thermal decomposition of the ammonium sulfate in a vessel different from that wherein the ammonium bisulfate reduction is performed.
According to another proposition (U.S. Pat. No. 3,676,059), the operation is performed by thermal decomposition at a temperature from 400.degree. C. to 510.degree. C. Unhappily, when using the latter technique, it is observed that the desired reaction: EQU 3(NH.sub.4).sub.2 SO.sub.4 .fwdarw.4NH.sub.3 +N.sub.2 +3SO.sub.2 +6H.sub.2 O (I)
and/or EQU 3NH.sub.4 HSO.sub.4 .fwdarw.NH.sub.3 +N.sub.2 +3SO.sub.2 +6H.sub.2 O (II)
is accompanied with an undesirable decomposition of the ammonium bisulfate (and/or ammonium sulfate): EQU NH.sub.4 HSO.sub.4 .fwdarw.SO.sub.3 +NH.sub.3 +H.sub.2 O (III)
This latter reaction even often prevails over the desired reaction. It has serious disadvantages: SO.sub.3 is recycled with ammonia, which increases the corrosion of the ducts. Moreover, this SO.sub.3 forms again ammonium sulfate during the cooling step and uselessly consumes a portion of the ammonia available for absorption of SO.sub.2, thereby reducing the efficiency of the absorption.
The process described in U.S. Pat. No. 3,695,829, comprising a two steps operation, suffers from the same disadvantages.